1. Field of the invention
The present invention relates to a ladder filter, and more particularly to a ladder filter which is preferably used for a transceiver, a cordless telephone and so on.
2. Description of Related Art
Conventionally, a ladder filter, as shown in FIG. 6, which is assembled by stacking a plurality of piezoelectric resonators and terminals and inserting the stack of resonators and terminals into a case is well known. The ladder filter of FIG. 6 is formed by laminating piezoelectric resonators 3, 4, 5 and 6, terminals 11, 12, 13 and 14 and a dummy terminal 15 and inserting the resonators and terminals into a resin case 1 which has an opening at the left side in FIG. 6. The piezoelectric resonators of this filter form an equivalent circuit as shown in FIG. 5. Since FIG. 5 shows a preferred embodiment of the present invention, the reference numerals of the preferred embodiment are indicated.
However, in the conventional ladder filter, both the resonators 3 and 4 connected in series and the resonators 5 and 6 connected in parallel operate in a stretch vibration mode. Thereby, the area and the thickness of each resonator 3, 4, 5 and 6 are to a specified size, and the height of each of the resonators becomes large. In addition, because the resonators 3, 4, 5 and 6 having a relatively large height are stacked vertically, the stack of resonators also has a great height thereby preventing the size of the filter from being reduced.